Cowboys' Defense Is A Top 5 Unit According to Aikman Efficiency Rating

The Cowboys defense has garnered all sorts of accolades over the  last few days, most recently when it was announced that Sean Lee had been named NFC Defensive Player of the Week.

And there's no denying that the defense has played very well over the last two games, especially when considering the players out injured or suspended.

Even the NFL picked up on this in their press release touting the Week 3 games.

The Cowboys boast the league’s top - ranked run defense (53 yards per game) and are third in overall defense (257.5 yards per game)

These two numbers are what we would call "vanity stats". Vanity stats are good for feeling awesome, but have little relevance for winning games.

In statistics, the relationship between two variables is called a correlation, and the strength of that correlation is measured by the "correlation coefficient". This coefficient (r²) is expressed as a number between 1 and -1. The closer the r² number is to 1 or -1, the stronger the relationship between the two variables. The closer it is to zero, the weaker the relationship.

In 2014, r² between Rushing Yards Allowed and wins in the NFL was 0.23, r² for total yards yards and wins was also 0.23. Both are pretty low numbers as far as predictive stats go.

Yet football is still very much dominated by volume based stats like total yards, points, number of completions, number of touchdowns and the like. While the major networks and many people who are paid to report about the NFL would have you believe that what matters is how many yards you accumulate, this is not in fact true. Volume stats do not correlate to victory. But efficiency stats do. How much someone passes or runs for can make for nice anecdotal discussions in the context of fantasy football, but has next to nothing to do with winning in the NFL.

But all of that doesn't mean the Cowboys defense is bad. Far from it. It's just that a lot of the commentary is focused on the wrong stats. Last year, Tom Cowlishaw of the Dallas Morning News wrote about which stats Rod Marinelli uses to assess his defense:

When I talked to Rod Marinelli in training camp, he said there were only two sets of numbers he cared about for his defense: the Aikman ratings and turnovers.

The Aikman ratings are a set of efficiency numbers (like a passer rating, but they encompass more things). The Cowboys ranked 32nd in that last year as well.

Now those are two stats that are much more interesting in terms of their correlation with winning in the NFL. Turnover differential had an r² in 2014 of 0.48, but even that solid number is overshadowed by the Aikman Efficiency Ratings, which had a spectacular r² of 0.84 in 2014. The only stat more closely correlated to wins is points differential, which comes in at 0.90.

The Aikman Efficiency Ratings were devised by - you guessed it - Troy Aikman, who was looking for a better way to rate offenses than the NFL’s "official" method of measuring offensive and defensive performance: yards gained or allowed. In 2006, together with Rick Odioso, a statistician for Fox Sports, and Jim Henzler, a senior analyst for Stats Inc., Aikman created the formula for the Aikman Efficiency Ratings (A.E.R.), which combines seven stats in five categories that Aikman believed offered a truer picture of offensive and defensive strength.

The exact formula used to arrive at the ratings was never made public, and remains proprietary to Stats Inc., but we do know what the seven stats are and how they are weighted in the formula:

Before looking at the overall AER numbers, let's walk through the individual metrics for the Cowboys defense after two weeks.

Adjusted Points: I'll assume that "returns" excludes not just punt and kick returns for a TD, but also interceptions and fumbles returned for a TD. Which would mean that of the 36 points the Cowboys have allowed over two games, seven points came from a fumble returned for a TD by the Giants. The 29 remaining points would rank the Cowboys 4th overall with 14.5 adjusted points allowed per game.

Turnovers: The Cowboys have three turnovers so far, which ranks them a joint 13th in the league.

Red Zone Efficiency. The Cowboys opponents have had six red zone possessions. At seven points per possession, opponents could have scored 42 points, but only scored 23 for an efficiency percentage of 55%, the sixth best value in the league.

Yards per play: The Cowboys are allowing 2.6 yards per attempt on the ground (1st) and 5.7 yards per attempt through the air (3rd).

First down achievement. The Cowboys have allowed 17.5 first downs per game, the 6th best value in the league. The Cowboys' third down percentage isn't quite as impressive but still very solid with a 32.0% conversion rate allowed, the 10th best value in the league.

While we don't know how the Aikman Efficiency Ratings are ultimately calculated, we do know that the stats above are combined into one single number and then adjusted against a league norm established over the last 10 years.

The resulting number is usually somewhere between 50 and 100, and the better the offense or defense, the higher the score on either scale. The average on offense is around 80 points, the average on defense is slightly below 70 points. Combine the two numbers and you get a value for overall team efficiency.

Here's what this year's Cowboys looks like versus last year's edition:

Overall, the Cowboys are ranked remarkably similar to last year after two weeks. The difference is that the defense has carried more of the load this year, while the offense hasn't really gotten into any kind of rhythm yet. Whether the offense will get back on track with Weeden under center is anybody's guess, but the defense is a highly functional unit. And a top five unit to boot.

Week 2 obviously is still a little early to take the AER numbers as gospel. But as we get more data, the numbers will become more robust, which is why this is one of the stats I'll be watching closely this season. For now though, it looks like the Cowboys are headed in the right direction.

The ratings are published weekly, usually on Wednesday, and you can find them on the homepage of Troy Aikman Enterprises.